1. Field of the Invention
The present invention relates to a method of forming synthetic resin powder paint coating on a prestressing strand used as tensioning members for a post-tensioning system or a pre-tensioning system of a prestressed concrete structure in a building structure or a civil engineering structure, i.e., a method of forming corrosion protection coating. In particular, it relates to a method of forming double coatings only on a surface layer part in case there is a fear of damage of corrosion protection coating on a prestressing strand in a special structure, and a prestressing strand obtained by this method.
2. Related Art
Generally, the prestressing strand has a structure in which fine surrounding wires are twisted around a core wire. This is for giving flexibility to the prestressing strand, and for obtaining adhesion strength to a concrete by means of helical groove parts formed by twisting the surrounding wires. Accordingly, also as a method of forming corrosion protection coating on the prestressing strand, there is desired a method which does not hinder the above properties. At present, several methods have become publicly known or well known as the method of forming corrosion protection coating on the prestressing strand.
A 1st prior art method utilizes a prestressing strand whose sectional shape is shown in FIG. 10. A method of forming corrosion protection coating for this prestressing strand is one in which first the prestressing strand is heated, surrounding wires 1b are temporarily untwisted from the circumference of a core wire 1a by a strand opener, the untwisted surrounding wires 1b untwisted are again returned to an original twisted state in a place where untwisted portions of the surrounding wires 1b enter into an electrostatic powder painting machine by 15 inches to 18 inches, a resin 50 during melting and adhering to (during gel time) the core wire 1a and the surrounding wires 1b is moved (caused to flow) to and filled in void portions between the core wire 1a and the surrounding wires 1b by twisting stresses of the surrounding wires 1b, and additionally, in order to prevent pinholes generated in the helical groove parts formed by twisting the surrounding wires 1b, a thick coating 51 (500-600 μm) is formed to make inner and outer parts monolithic, thereby obtaining a composite body (U.S. Pat. No. 5,208,077).
Further, a 2nd prior art method utilizes a prestressing strand whose sectional shape is shown in FIG. 11. The method of forming corrosion protection coating for this prestressing strand is one in which, after a surface preparation, the surrounding wires 1b of the prestressing strand are temporarily untwisted in order from the circumference of the core wire 1a by use of a loosening-and-untwisting device, the surrounding wires 1b are kept apart from the core steel wire 1a in a spacing necessary for a next process by a wire expander, the core wire 1a passes through a core-length adjusting device, and a synthetic resin powder paint is individually sprayed onto the whole outer peripheral face of each of the core wire 1a and the surrounding wires 1b by an electrostatic painting method and adhered by an electrostatic repulsive force, thereby forming a resin coating 52. It is a method of forming corrosion protection coating in which the powder paint adhered by the electrostatic repulsive force is heated and molten, forms the individual resin coating 52 by cooling after elapse of the gel time and a curing and standing time, and thereafter the untwisted surrounding wires 1b are twisted with respect to the core wire 1a to the original state by a tightening device (U.S. Pat. No. 5,362,326).
In the prestressing strand formed in this manner, since the coating is individually formed one by one over the whole outer peripheral face of each of the core wire 1a and the surrounding wires 1b, which is different than 1st prior art method, the properties, such as the flexibility and the adhesion strength to the concrete, demanded as the prestressing strand are not hindered at all and, moreover, a corrosion protection function is sufficient, so that it is evaluated that this corrosion protection method is an ultimate corrosion protection method for the prestressing strand.
Further, the prestressing strand in which the individual corrosion protection coating is formed on the whole outer peripheral face of each of the core wire 1a and the surrounding wires 1b by the corrosion protection method according to the 2nd prior art is excellent also in its tensile strength, and this excellent property conspicuously appears especially in a case where a stress amplitude is large. One example of test results when it is subjected to tensile fatigue tests under the same conditions as a usual prestressing strand before the corrosion protection working was as shown below.
TABLE 1Tensile fatigue test results (specification value 2 × 106 times)UpperLower limitlimitstressTest resultsstress(Pu × 0.45 −StressPressure(Pu × 0.45)25)amplitudeFinalNumber ofbondingKind ofσ maxσ minΔσnumber ofrupturedgripprestressingKgf/mm2Kgf/mm2Kgf/mm2repetitionsstrand(s)deformationstrand(tf)(tf)(tf)NPiece(s)ExistencePrestressing186(12)61(8.5)25(8.5)21.0 × 1042nonestrand before286(12)61(8.5)25(8.5)28.3 × 1041nonecorrosion386(12)61(8.5)25(8.5)36.6 × 1043noneprotectiontreatment(15.2 mm)Method of186(12)61(8.5)25(8.5) 400 × 104no rupturenoneUS-A-5362326286(12)61(8.5)25(8.5) 400 × 104no rupturenone(15.2 mm)386(12)61(8.5)25(8.5) 400 × 104no rupturenone
As apparent from the above test results (Table 1), it is understood that, among the general prestressing strand to which no corrosion protection treatment is applied and the prestressing strand which is described in U.S. Pat. No. 5,362,326 in which the individual corrosion protection coating is formed on the whole outer peripheral face of each of the core wire and the surrounding wires, the one in which the corrosion protection coating is formed is remarkably improved with respect to its tensile strength.
As a main factor of this, the fact is recognized that, by forming the individual coating on the whole outer peripheral face of each of the core wire and the surrounding wires, a portion where a metal-to-metal contact occurs is completely nullified, so that it becomes possible to prevent fretting corrosion, contact corrosion and the like. With such a corrosion protection method, not only is a corrosion protection function remarkably improved but also the tensile strength is remarkably improved. Accordingly, in this prestressing strand, in the case where the individual coating is formed on the whole outer peripheral face of each of the core wire and the surrounding wires, it is desirable that a thickness of the coating of each of the core wire and the surrounding wires is made about 250 μm of a range in which a helical constitution of the twisted surrounding wires is stably held and a twisted state is sufficiently maintained.
In the industry, a regulation of the thickness of this kind of coating is accomplished as follows in outline. Namely, according to many research results, it is reported that, in order to satisfy a corrosion resistance performance and dynamic performances (impact resistance, bending property, and ability to adhere to concrete), the coating thickness of 200±50 μm is adequate if a powder type epoxy resin painting is adopted. Further, also in experimental results of the FHWA (Federal Highway Administration) in the U.S.A., it is reported that a range of 170±50 μM is desirable.
Additionally, an article to be painted with a coating thickness under this regulation is “Steel Bar for Ferroconcrete under JIS G 3112 (Japanese Industrial Standards)” (deformed steel bar), and is one completely different from a round steel bar. And, it is one having protrusions (ribs) on its surface in an axial direction, and having protrusions (nodes) also in a direction other than the axial direction, so that the above regulation of the coating thickness is determined by sufficiently considering the fact that the article to be coated has a structure in which, in the protrusion portions, there are many corner places where the powder paint is difficult to adhere.
Accordingly, in a case of a simple round steel bar shape like the core wire and the surrounding wires in the prestressing strand, since the powder paint evenly adheres to its outer peripheral face, it is needless to say that there is no problem so long as the coating thickness is 200±50 μm.
Additionally, a 3rd prior art method utilizes a prestressing strand whose sectional shape is shown in FIG. 12. This prestressing strand is made for a case where there is a fear that the corrosion protection coating will be damaged by a special structure and thus a maximum coating thickness of 250 μm or more by which the coating can be stably held is demanded, and a double coating corrosion protection working is performed, with respect to the prestressing strand of the 2nd prior art, by additionally forming a thick resin coating 53 on its outer peripheral face (JP-A-11-200267)
In the 1st prior art, since it is the prestressing strand made monolithic in which the resin powder is moved (caused to flow) to and filled in the void portions between the core wire and the surrounding wires by the stresses twisting the surrounding wires during when the resin powder is molten and adhered to (during gel time) the core wire and the surrounding wires and the thick coating is formed also in the surface layer part, the flexibility demanded to the prestressing strand cannot be expected at all. Further, since it is not only impossible to expect an improvement in the tensile strength, but also the helical groove part due to twisting the surrounding wires becomes shallow, there arises a problem that the adhesion strength to the concrete is reduced.
Additionally, this prestressing strand is one in which the resin is filled in the internal spaces. However, it has a structure in which basis surfaces still contact each other in contact portions between the core wire and the surrounding wires and between the mutual surrounding wires, so that no corrosion protection coatings are formed between the core wire and the surrounding wires and between the mutual surrounding wires, and thus it cannot be said that a problem of so-called internal corrosion is solved.
Further, in the 2nd prior art method, the structure has the individual resin coating formed in each of the core wire and the surrounding wires of the prestressing strand. It is possible to expect improvements in the flexibility and the tensile strength demanded of the prestressing strand. However, in its corrosion protection coating formation process, the surrounding wires are twisted with respect to the core wire to the original state after the individual resin coating has been formed on each of the core wire and the surrounding wires, and the thickness of the resin coating individually formed is about 250 μm and thus it cannot be made so thickly, there is a problem that it cannot be used in such a situation or place that there is the fear that the corrosion protection coating will be damaged by the special structure and thus a thick coating is demanded in order to prevent an exposure of the basis surface by the damage of the coating.
Additionally, in the 3rd prior art, the thick coating is formed in the outer peripheral face of the prestressing strand by applying the double coating corrosion protection. However, the flexibility demanded of the prestressing strand is hindered by the thick coating formed in the outer peripheral face, and not only the tensile strength is hindered to no small extent but also the adhesion strength to the concrete is reduced because the helical groove parts in the outer peripheral face become shallow.
Accordingly, in the prior art methods, there are such problems to be solved that the improvement in the tensile strength should be intended so as not to impair the flexibility and the adhesion strength to the concrete demanded of the prestressing strand, and that the thick coating should be formed in the surface layer part (outer peripheral face) in order to prevent the exposure of the basic surface by the damage of the coating.